Greening of coffee waste through its transformation into clean and structurally stable activated carbon for energy storage applications

Abstract

The sustainable transformation of biomass waste into high-purity activated carbon (AC) offers a promising solution to environmental challenges while advancing material innovations. However, conventional methods often yield materials with limited purity and performance. This study presents a scalable process to produce high-purity AC (99.7 wt%) from spent coffee grounds using optimized alkaline infusion, acid etching, and thermal treatment. The process achieves near-complete removal of inorganic residues and metal oxides, significantly improving structural, compositional, and electrochemical properties. Alkaline infusion with a 1 : 1.5 biochar-to-NaOH ratio reduced ash content from ∼7.8 to <0.3 wt% and increased carbon content to 83.6 wt%. Acid etching further removed residual oxides such as potassium and sodium, achieving 99.7 wt% purity. The surface area increased from 1.5 m² g⁻¹ (biochar) to 550 m² g⁻¹ (purified AC), with well-developed micropores (∼1.6 nm) and mesopores. As a proof of concept, the purified AC achieved a specific capacitance of 49 F g⁻¹ at 0.5 A g⁻¹, 59.6% higher than commercial AC (30.7 F g⁻¹). Further thermal treatment at 1500 °C reduced oxygen content to 1.5 wt% while increasing carbon to 98.5 wt%. These results establish coffee waste-derived AC as a scalable, high-performance alternative for energy storage and circular economy applications.